Project Summary/Abstract Evidence from clinical trials and animal experiments shows a single subanesthesia dose of ketamine produces rapid antidepressant responses. However, the psychotomimetic properties and abuse potential of ketamine necessitate caution in promoting this compound to be used as a general treatment of depression. The reports on the mechanisms underlying the rapid-acting antidepressant effects of ketamine are controversial. Particularly, it remains uncertain which molecular events mediate ketamine?s or its metabolites? action in enhancing excitatory synaptic transmission. In preliminary experiments, it was observed that both ketamine and its metabolite, cis-6-HNK, potently enhanced Shaffer collateral-CA1 (SC-CA1) EPSCs and fEPSPs. These actions were mimicked and completely occluded by MK-801, but not eliminated by GABA receptor blockade. Ketamine potently increased total and surface GluA1 expression and the phosphorylation of GluA1 Ser845 site. In addition, ketamine failed to potentiate SC-CA1 fEPSPs and induce the antidepressant-like responses in GluA1 S845A knock-in mice. Furthermore, deletion of NMDA receptor on CA3 neurons but not on CA1 cells completely eliminated ketamine-induced potentiation on SC-CA1 synaptic transmission. In the proposed project experiments are designed to test the role of GluA1 Ser845 phosphorylation and presynaptic NMDA receptors in ketamine- and HNK-produced rapid-acting antidepressant responses as well as their celluar and molecular bases. Electrophysiological techniques will be combined, including whole-cell patch clamp with cell biological technique such as Western blotting, biotinylation of surface proteins and GluR1 S845A knock-in mice, CA1 cell- or CA3 cell-specific NR1 knockout mice, HCN1 knockout mice as well as behavioral measurements to achieve the designed experiments. The proposed project will expand knowledge in understanding the cellular mechanisms of fast-acting antidepressant actions of ketamine. A better understanding of ketamine?s fast-acting antidepressant effects will promote the design of a new generation of rapid-acting antidepressant to offer a better therapy to depression.